Vehicle braking unit

Claim 1

BRAKING MECHANISM, COMPRISING: (A) COOPERABLE BRAKING ELEMENTS MOUNTED FOR RELATIVE MOVEMENTS INTO AND OUT OF ENGAGEMENT; (B) A CYLINDER; (C) A MAIN PISTON FOR ACTUATING ONE OF SAID BRAKING ELEMENTS; (D) AN AUXILIARY PISTON HAVING A LONGITUDINAL AXIS COAXIAL WITH THE MAIN PISTON AXIS, SAID PISTONS HAVING ADJACENTLY POSITIONED INNER ENDS; (E) AXIALLY SPACED ABUTMENTS CARRIED BY THE AUXILIARY PISTON ADJACENT ITS INNER END; (F) AN ANNULAR STOP MEMBER CARRIED BY SAID MAIN PISTON SURROUNDING THE AUXILIARY PISTON AND POSITIONED FOR LIMITED MOVEMENT BETWEEN SAID ABUTMENTS TO ESTABLISH A PREDETERMINED CLEARANCE; (G) A COMPRESSION SPRING EXTENDING BETWEEN THE INNER END OF SAID AUXILIARY PISTON AND SAID STOP MEMBER; (H) MEANS FOR APPLYING A FLUID PRESSURE BETWEEN THE INNER ENDS OF THE PISTONS, SAID PRESSURE ACTING EACH TIME IT IS APPLIED TO RELATIVELY MOVE THE PISTONS IN OPPOSITE DIRECTIONS TO THE EXTENT OF SAID LIMITED MOVEMENT AND ESTABLISH SAID PREDETERMINED CLEARANCE; AND (I) A HOLDING DEVICE FOR RELEASING SAID AUXILIARY PISTON FOR MOVEMENT RESPECTIVELY IN OPPOSITE DIRECTIONS WHEN A PREDETERMINED HOLDING FORCE IS EXCEEDED, SAID HOLDING DEVICE INCLUDING (A) A RING MEMBER HAVING PRESET FRICTIONAL GRIPPING RELATION WITH SAID AUXILIARY PISTON.